AbstractThe unprecedentedly growing demand for energy storage devices in recent years calls for diversified chemistries with unique advantages. When it comes to safety and cost, aqueous battery systems have attracted tremendous attention. Owing to its small size, high polarity, and hydrogen bonding, water in the electrode materials, either in the form of structural water or cointercalated hydrated cations, drastically change the electrochemical behavior through multiple aspects. This review discusses the roles of water in aqueous batteries from how water molecules coordinate with cations to examples of water‐mediated reactions in different types of host materials.